Visa Program Struggles to Track Missing Foreign Students

The Department of Homeland Security has lost track of more than 6,000 foreign nationals who entered the United States on student visas, overstayed their welcome, and essentially vanished -- exploiting a security gap that was supposed to be fixed after the Sept. 11, 2001 terror attacks.

"My greatest concern is that they could be doing anything," said Peter Edge, the U.S. Immigration and Customs Enforcement official who oversees investigations into visa violators. "Some of them could be here to do us harm."

Homeland Security officials disclosed the breadth of the student visa problem in response to ABC News questions submitted as part of an investigation into persistent complaint